TBar, that may well be the answer, the drivers after v301.42 include CUDA 5, so it may well be an incompatibility with the processors Intel(R) Pentium(R) III CPU - S 1400MHz [Family 6 Model 11 Stepping 4] (2 processors) and CUDA 5, although as you say there is no mention of CPU requirements.

Claggy, I've not been able to run GPUCapsViewer as it crashes out each time I attempt to run it. However I've run GPU-Z against the v301.42 drivers and it reports CUDA & OpenCL as available. I've lost the connection to the computer (VNC) before trying it with v310.90, and won't be able to restore the connection until Friday when I can get to the computer in person!!

I have had similar issues when I tried upgrading the boinc version on my rig from 6.10.60 to 7.0.28. I have been running nVidia driver 306.81 for a while now. Plus the issue is certainly not due to the processor as I have the newer AMD Phenom II X6 1055T. I have now reverted to 6.10.60 and all is well.

You might want to Stick to 301.42, my testing of 306.xx and later drivers on my 9800GTX+ showed a slowdown on the different x41zc Cuda apps, with the exception of the Cuda23 and Cuda22 apps,
In the meantime i've got my XP3200+/AGP4650/PCI8400GS running, and done some MB v6 benches, as you can see the Cuda23 x41zc app is fastest on most of the Wu's, so no need to upgrade drivers further:
(Note: the x41zc_Cuda22 times are misleading because it was run with Cuda23 dll's, normally it would be a lot slower.)